A ceramic honeycomb structure (simply called “honeycomb structure” below) used as a filter for cleaning an exhaust gas, etc. is produced by extruding a moldable ceramic material through a honeycomb-structure-molding die (simply called “molding die” or “die”) to form a honeycomb molding, and drying and sintering it. As shown in FIGS. 2(a) and 2(b), a honeycomb structure 10 comprises porous cell walls 2 defining large numbers of flow paths 3 and an outer peripheral wall 1, and usually has a substantially circular or elliptic cross section perpendicular to the flow paths.
As shown in FIGS. 3(a) and 3(b), a die 20 for extruding a moldable ceramic material to form a honeycomb molding is formed from a die-forming work 21 having such a shape that a groove-having surface 31 projects, that moldable-material-supplying apertures 40 are formed in the die-forming work 21 such that they extend from an aperture-having surface 41, and that lattice-patterned grooves 30 are formed on the groove-having surface 31. As shown in FIG. 4, the apertures 40 are overlapping the intersections 33 of the grooves 30 in the molding die 20. A moldable ceramic material introduced into the molding die 20 through the apertures 40 are formed into a honeycomb shape by the grooves 30. The apertures 40 are communicating with the grooves 30 at every intersections 33 of the grooves 30 arranged in a lattice pattern, or at every other intersections 33 (see FIG. 4).
The molding die 10 is produced by forming apertures 40 by drilling, etc. in the die-forming work 21 having a projecting groove-having surface 31 as shown in FIG. 5(a) from its aperture-having surface 41 (on the opposite side of the groove-having surface 31), and then forming the grooves 30 on the groove-having surface 31. The lattice-patterned grooves 30 are produced by forming pluralities of first parallel grooves 30a by grinding or cutting by a rotating tool 50 as shown in FIG. 5(b), rotating the die-forming work 21 by 90°, and then forming pluralities of second parallel grooves 30b crossing the previously formed first grooves 30a as shown in FIG. 5(c).
In a conventional extrusion-molding die, a groove-having surface 31 having an outer periphery 32, which is square as it is formed when viewed from above as shown in FIG. 3(a), or made circular by machining as shown in FIG. 6 as described, for instance, in JP 11-70510 A. In any case, a moldable ceramic material extruded from the die 20 is caused to pass through a guide ring 62 as shown in FIG. 7, to obtain a honeycomb molding 11 having a substantially circular or elliptic cross section perpendicular to the flow paths.
However, when the outer periphery 32 of the groove-having surface 31 has a square shape as shown in FIG. 3(a), some of the grooves 30 which are not used for the extrusion of a honeycomb molding having a substantially circular or elliptic cross section perpendicular to the flow paths are also formed by machining, failing to reduce the number of machining steps. This is true, even when a die having a groove-having surface 31 having a square outer periphery 32 as shown in FIG. 3(a) is formed by machining, and the outer periphery 32 of the groove-having surface 31 is then machined circularly as shown in FIG. 6, as described in JP 11-70510 A.
Using a die-forming work 21 having a groove-having surface 31 having a circularly formed outer periphery 32 to form grooves 30 by machining, the above inefficiency of machining some grooves 30 that are not used for extrusion can be avoided. However, in the machining of the groove-having surface 31 having a circular outer periphery 32 to form grooves 30, some grooves 30 are at extremely small angles 70 to the outer periphery 32 of the groove-having surface 31 as shown in FIG. 8. When machining starts to form a groove 30 as shown by the dotted line in FIG. 8, which has an extremely small angle 70, a cutting edge of a rotating tool 50 is likely to slide along the outer periphery 32 of the groove-having surface 31 as the rotating tool 50 progresses, so that a force P is applied to the tool 50 in its thickness direction (rotation axis direction), deforming or breaking the tool 50.